The present invention relates to gear-type devices, such as transmissions and axle assemblies, which include at least one, but typically, a plurality of gears rotatable about an axis of rotation of a shaft.
In many types of gear devices, in which the gears mounted about a shaft are in meshing engagement with other gears, it is necessary to provide some sort of lubricant fluid to lubricate the gear mesh. The lubricant fluid minimizes the generation of wear particles, and dissipates frictional heat generated in the gear mesh. In many such gear devices, the housing which surrounds the gears acts both as a reservoir for lubricant fluid, and as a heat transfer surface, and accordingly, it has been common practice to put enough lubricant within the housing so that each gear rotating about the shaft will be rotated through the fluid lubricant, and pick up some of the lubricant on the teeth of the gear, where some of it remains until those particular teeth come into meshing engagement with the teeth of a mating gear.
Although providing sufficient lubricant in the reservoir, such that the teeth of each gear along the shaft will pass just below the fill level of the lubricant, has proven to be generally satisfactory, in terms of the ability to lubricate, there are certain disadvantages to the arrangement described.
By way of example only, in a typical multi-forward-speed, change gear transmission of the countershaft type, there is typically a mainshaft and two countershafts, an "upper" countershaft and a "lower" countershaft. On each countershaft, there are anywhere from perhaps four countershaft gears to as many as perhaps seven countershaft gears disposed axially along the countershaft. In such a device, each countershaft gear is associated with a different speed ratio, and therefore, each gear has a substantially different diameter. The use of the typical lubrication method, described above, requires that the lubricant level in the transmission housing be high enough for the teeth of the smallest diameter gear to pass through the lubricant, as the gear rotates.
The disadvantage of such an arrangement is that the largest gear on the countershaft, and perhaps the largest two or three gears on the countershaft, rotate through lubricant which is much deeper than is truly needed, merely to lubricate the teeth of those particular gears. In fact, it has been determined that the teeth of those larger gears, whose teeth have the greater linear velocity, rotating through a fairly deep reservoir of lubricant fluid, can result in substantial "churning losses", i.e., transmission horsepower which is consumed, and therefore lost, in turning the countershaft, and merely overcoming the resistance offered, by the lubricant, to the rotation of the gears.
It has been found that there are actually some benefits to slightly "overfilling" the transmission housing with lubricant fluid. Such overfilling provides a "margin of error" which permits operation of the vehicle on a grade, or permits a non-level mounting angle for the transmission. Also, overfilling with lubricant provides some allowance for leakage of fluid during operation. However, overfilling also has the effect of further increasing the "churning losses" discussed above, and therefore, in spite of the potential benefits of overfilling, it has not been common to do so because of the overall decrease in transmission efficiency which would result from the increased churning losses.